Virulent combinations of adhesin and toxin genes in natural populations of Staphylococcus aureus.

1Nuffield Department of Clinical Laboratory Sciences, John Radcliffe Hospital, University of Oxford, United Kingdom. sharon.peacock@ndcls.ox.ac.uk

Abstract

Most cases of severe Staphylococcus aureus disease cannot be explained by the action of a single virulence determinant, and it is likely that a number of factors act in combination during the infective process. This study examined the relationship between disease in humans and a large number of putative virulence determinants, both individually and in combination. S. aureus isolates (n = 334) from healthy blood donors and from patients with invasive disease were compared for variation in the presence of 33 putative virulence determinants. After adjusting for the effect of clonality, seven determinants (fnbA, cna, sdrE, sej, eta, hlg, and ica) were significantly more common in invasive isolates. All seven factors contributed independently to virulence. No single factor predominated as the major predictor of virulence, their effects appearing to be cumulative. No combinations of the seven genes were either more or less likely to cause disease than others with the same number of virulence-associated genes. There was evidence of considerable horizontal transfer of genes on a background of clonality. Our findings also suggested that allelic variants of a polymorphic locus can make different contributions to the disease process, further study of which is likely to expand our understanding of staphylococcal disease pathogenesis.

Presence or absence of putative virulence determinants. Odds ratios for disease, adjusted for the effects of clonality by Mantel-Haenszel stratification of MLST-defined lineage. Error bars denote 99% confidence intervals (CIs), and an asterisk indicates a significant association with disease (P < 0.01). Genes that were either ubiquitous (such as clfA and clfB) or very rare or absent (such as see and etb) are not shown.

(A) Association between the number of virulence-associated determinants and the proportion of isolates from cases of disease (rather than carriage). For example, 20% of isolates with one virulence-associated gene were from cases of disease and 80% were carriage strains. The area of each circle is proportional to the number of isolates with that number of determinants (ranging from zero genes to seven genes, with n = 2, 15, 21, 64, 140, 56, 22, and 7, respectively). (B) Association between the number of virulence-associated determinants in a gene combination and the proportion of isolates with that combination from cases of disease (rather than carriage). Each combination is identified by a letter; for the actual gene complement, refer to Table . The 12 most common combinations representing 80% of isolates are shown. The area of each circle is proportional to the number of isolates in that gene combination (n for each gene combination is given in Table ).

Distribution of genes within the seven largest MLST-defined clonal complexes (denoted by numbers along the x axes in the bottom row). The overall height of each bar denotes the total number of isolates in the complex. The height of the shaded area represents the number of isolates positive for the determinant. The population genetic structure of S. aureus can be found on the MLST website (http://www.mlst.net).